I've tried Avast, Avira, Panda, and am now giving MSE a try. This old box doesn't like any of them. My personal XP has about the same specs as this one, except my processor isn't as fast as this one, and, like you, mine handles them all (one at a time, of course) with no problems. Seems like something else is "bothering" it, but I can't find out what it is.